The underlining unit for punctuation marks.

Underlining's a line that is under,
A sort
Of a floor that's supporting a word.
Long or short,
Or to tightrope a word that seems to walk,
And no botching.
All eyes are upon it, and everyone's watching.

Underlining is the thing to use to suggest
That's this word
Or that word's
The word to be stressed,
Or to show it's a title,
Like Better Expression,
But it can be—like dashes—
A writer's obsession.
Too much underlining, all over the place,
Will often bring lines to the poor reader's face.

So though it is useful and though it is fun,
Be sure underlining
Is not overdone.

This _____ is used to mark with a line underneath a word or several words, if the user wants to emphasize something on the computer; or when hand writing with a pen, pencil or other device; for titles of books, other publications, or to draw special attention to a word or groups of words.

For more information, see this page at Underlining.